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UV Surface Habitability of the TRAPPIST-1 System [EPA]

With the discovery of rocky planets in the temperate habitable zone (HZ) of the close-by cool star TRAPPIST-1 the question of whether such planets could also harbour life arises. Potentially habitable planets around red dwarf stars can orbit in radiation environments that could cause life-sterilizing surface radiation events. UV flares from these stars are more frequent and intense than solar flares. In addition, their temperate HZs are at closer separations from the star. Here we show UV surface environment models for planets in the HZ of TRAPPIST-1 and explore the implications for life. TRAPPIST-1 has high X-ray/EUV activity, placing planetary atmospheres at risk from erosion over time. If a dense Earth-like atmosphere with a protective ozone layer exists on planets in the HZ of TRAPPIST-1, UV surface environments would be similar to Earth. However an anoxic atmosphere, or an eroded thinner atmosphere, would allow more UV to reach the surface, making surface environments hostile even to highly UV-tolerant terrestrial extremophiles. If future observations detect ozone in the atmospheres of any of the planets in the HZ of TRAPPIST-1, these would be interesting targets for the search for life. We anticipate our assay to be a starting point for in-depth exploration of stellar and atmospheric observations of the TRAPPIST-1 planets to constrain their UV-surface-habitability.